1. Field of the Invention
The present invention relates generally to integrated circuits and logic devices. In particular, the present invention relates to digital logic devices used as counters and generators.
2. Description of Related Art
There are many instances when low noise counters are required. A major source of noise in digital electronics is the simultaneous switching of several outputs at the same time. The simultaneous switching of several outputs is a common occurrence in binary counters where many of the outputs may change if the counter is clocked (e.g., when a binary counter transitions from the highest possible number to zero). When low noise counters are required, Gray code counters are typically used. Gray code counters advantageously change only one output or bit each time the counter is clocked or incremented. Thus, the noise from the simultaneous switching of several outputs is eliminated.
While the Gray code counters of the prior art have attempted to eliminate the noise problems of normal binary counters, the prior art Gray code counters have significant disadvantages. Some of the counters and generators of the prior art are disclosed in U.S. Pat. Nos. 3,209,347; 3,349,332; 3,393,298; 3,588,461; 3,600,686; 4,119,961; 4,264,807; 4,596,979; 4,618,849; and 4,780,894. A particular problem in the prior art is that Gray code counters are typically constructed using a combination of binary counters and Exclusive-OR gates. For example, the making of a nine bit Gray code counter commonly requires three 4-bit binary counters (74F161) and two quad two-input Exclusive-OR gates (74F86). Thus, a nine bit Gray code counter effectively requires five separate integrated circuits to be implemented. The use of five integrated circuits consumes more power, and requires a significant amount of space. Similarly, the teachings in U.S. Pat. No. 3,209,347 issued to Cutaia require 210 parts to construct a nine bit Gray code counter. The large number of parts increases the amount of power required and the size of the counter. Additionally, having to interconnect so many parts decreases the reliability of such Gray code counters. In applications such as aerospace, missiles and spacecraft where size and weight are at a premium, there is a need for a nine bit Gray code counter with reduced size and weight.
Another problem with the Gray code counters of the prior art is that noise is not completely eliminated. Since Gray code counters are often built from binary counters, the noise from several outputs of the binary counter changing simultaneously is propagated through the Exclusive-OR gates that often provide the outputs for the Gray code counter. For example, U.S. Pat. Nos. 4,119,961 and 4,618,849 disclose designs employing binary counters. While this problem may be eliminated by buffering the outputs of the logic gates with registers so that only one output will change, the addition of registers only adds to the size and weight of such Gray code counters. Additionally, the use of buffer registers also increases the complexity of the timing required for clocking the registers. Noise is also a problem in the prior art because the external clock signal must be coupled to several devices. Such coupling increases the signals that may be affected by the clock signal. Other signals lines in close proximity to the clock signal may experience interference from the clock signal which creates noise.
As noted above, Gray code counters are conventionally constructed from parts such as simple logic gates that can perform only a single function, and cannot be reprogrammed as other electrical circuits. For each part used, a stock pile of replacement parts must be maintained. Thus, there is a need for a Gray code counter that may be constructed from components already used in missile and satellite applications, thereby eliminating the need to stockpile any additional parts.